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><A
NAME="TUTORIAL-INHERITANCE"
>3.6. Inheritance</A
></H1
><P
> Inheritance is a concept from object-oriented databases. It opens
up interesting new possibilities of database design.
</P
><P
> Let's create two tables: A table <CODE
CLASS="CLASSNAME"
>cities</CODE
>
and a table <CODE
CLASS="CLASSNAME"
>capitals</CODE
>. Naturally, capitals
are also cities, so you want some way to show the capitals
implicitly when you list all cities. If you're really clever you
might invent some scheme like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE capitals (
name text,
population real,
altitude int, -- (in ft)
state char(2)
);
CREATE TABLE non_capitals (
name text,
population real,
altitude int -- (in ft)
);
CREATE VIEW cities AS
SELECT name, population, altitude FROM capitals
UNION
SELECT name, population, altitude FROM non_capitals;</PRE
><P>
This works OK as far as querying goes, but it gets ugly when you
need to update several rows, for one thing.
</P
><P
> A better solution is this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE cities (
name text,
population real,
altitude int -- (in ft)
);
CREATE TABLE capitals (
state char(2)
) INHERITS (cities);</PRE
><P>
</P
><P
> In this case, a row of <CODE
CLASS="CLASSNAME"
>capitals</CODE
>
<I
CLASS="FIRSTTERM"
>inherits</I
> all columns (<TT
CLASS="STRUCTFIELD"
>name</TT
>,
<TT
CLASS="STRUCTFIELD"
>population</TT
>, and <TT
CLASS="STRUCTFIELD"
>altitude</TT
>) from its
<I
CLASS="FIRSTTERM"
>parent</I
>, <CODE
CLASS="CLASSNAME"
>cities</CODE
>. The
type of the column <TT
CLASS="STRUCTFIELD"
>name</TT
> is
<TT
CLASS="TYPE"
>text</TT
>, a native <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
type for variable length character strings. State capitals have
an extra column, <TT
CLASS="STRUCTFIELD"
>state</TT
>, that shows their state. In
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, a table can inherit from
zero or more other tables.
</P
><P
> For example, the following query finds the names of all cities,
including state capitals, that are located at an altitude
over 500 feet:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, altitude
FROM cities
WHERE altitude > 500;</PRE
><P>
which returns:
</P><PRE
CLASS="SCREEN"
> name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953
Madison | 845
(3 rows)</PRE
><P>
</P
><P
> On the other hand, the following query finds
all the cities that are not state capitals and
are situated at an altitude of 500 feet or higher:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, altitude
FROM ONLY cities
WHERE altitude > 500;</PRE
><P>
</P><PRE
CLASS="SCREEN"
> name | altitude
-----------+----------
Las Vegas | 2174
Mariposa | 1953
(2 rows)</PRE
><P>
</P
><P
> Here the <TT
CLASS="LITERAL"
>ONLY</TT
> before <TT
CLASS="LITERAL"
>cities</TT
>
indicates that the query should be run over only the
<CODE
CLASS="CLASSNAME"
>cities</CODE
> table, and not tables below
<CODE
CLASS="CLASSNAME"
>cities</CODE
> in the inheritance hierarchy. Many
of the commands that we have already discussed —
<TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, and
<TT
CLASS="COMMAND"
>DELETE</TT
> — support this <TT
CLASS="LITERAL"
>ONLY</TT
>
notation.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Although inheritance is frequently useful, it has not been integrated
with unique constraints or foreign keys, which limits its usefulness.
See <A
HREF="ddl-inherit.html"
>Section 5.8</A
> for more detail.
</P
></BLOCKQUOTE
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